1. Field of the Invention
The present invention relates to a compensation circuit for quadrature amplitude modulation (QAM) signals and more particularly to a circuit for compensating for any phase errors in the I and Q components of QAM signals to reduce carryover of such phase errors in analog up and down conversions of such signals.
2. Description of the Prior Art
Various modulation techniques are known for modulating a carrier signal with various types of information. Due to limited bandwidth allocations in some applications, modulation techniques have been developed to increase the amount of information that can be transmitted per frequency. One such technique is known as quadrature phase shift keying (QPSK). Such QPSK modulation techniques are known in the art and described in U.S. Pat. Nos. 5,440,259; 5,615,230; 5,440,268; 5,550,868; 5,598,441; 5,500,876; and 5,485,489, hereby incorporated by reference. In general, with such a modulation technique, the phase of both the real and quadrature (I-Q) components of the carrier are modulated in order to enable two bits, each having two states, to be transmitted over a single frequency. As such, at each frequency, the carrier can be modulated into one of four different states, known as symbols, which form what is known as a constellation. The QPSK modulation technique is thus able to provide twice the information per frequency, relative to other amplitude and frequency techniques, making it suitable for applications where bandwidth allocations are relatively limited, for example, in satellite communications systems.
In order to further increase the amount of information transmitted per frequency, other modulation techniques have been developed, such as quadrature amplitude modulation (QAM). Such QAM modulation techniques are relatively well known in the art. Examples of such QAM modulation circuits are disclosed in commonly owned copending U.S. patent application Ser. No. 09/175,790, filed on Oct. 20, 1998, as well as U.S. Pat. Nos. 5,612,651; 5,343,499; 5,363,408; and 5,307,377, hereby incorporated by reference. Such QAM modulation techniques essentially involve amplitude modulation of the QPSK signal to provide constellations of symbols of 8, 16, 32 and 64 and more per frequency.
Such QPSK and QAM modulation techniques are used for transmission of information at RF frequencies in various communication systems, such as military and commercial communication systems. In such systems QAM modulated signals are known to be up converted from baseband to RF frequencies for transmission. Unfortunately, any phase error in the I-Q components of the QAM signals, for example, due to hardware imperfections, results in I-Q cross talk, known to degrade the signal-to-noise ratio (SNR) of the transmitted signal. Similarly, down conversion of phase modulated signals from RF frequencies to baseband are also subject to I-Q cross talk which also results in a degraded SNR of the received signals. In addition, if the up or down conversions are accomplished utilizing analog techniques, for example, as in analog radios, additional problems may result due to equipment aging and temperature drift, which can cause the orthogonality of the I-Q components to vary, possibly degrading the conversion process. Thus, there is a need to compensate phase errors in the I and Q components of QAM modulated signals.